The present invention relates to apparatus for determining the instantaneous power in a phase of an inverter with a given input d-c voltage, especially in a pulsed inverter, as well as to its further development for determining the active and/or reactive power, the active and/or reactive current in this phase or for determining these quantities for the entire inverter.
Frequently, the measured value of the active or reactive current is used for damping, for instance, current and speed oscillations of inverter-fed asynchronous machines, where, a determination of the applicable measurement value with as little delay as possible is desirable.
One possibility is, for instance, to recalculate the output voltage into two-phase components and then to smooth and normalize them by means of a vector filter. The normalized output voltage is then multiplied by the corresponding two-phase components of the active and reactive current, respectively, or, if the unnormalized output voltage is used, the measured values of the active or reactive power. This requires not only appropriate computing stages for recalculation into two-phase components, but also the vector filter itself. If such measured values are required individually for the phases of the inverter, they must be calculated from the two-phase components.